568 Geological Society. 
The period of the finai desiccation of these river-lakes, and the reduc- 
tion of the rivers to their present channel, is supposed to have been 
contemporaneous with that recent elevation which in raising the land 
to greater heights brought up large adjacent portions of the bottom of 
the sea, and to the consideration of which the second part of the me- 
moir is devoted under the head of ‘ northern drift.” 
In the region of Welsh and local drift attention is specially called 
to the length of time during which existing causes have been in un- 
disturbed action, as proved by the magnificent mass of Travertino 
formed and still forming at the Southstone rock ; whilst he also points 
to the discovery of shell marl in a bog near Montgomery, containing 
several species of Lymnea, which has evidently been formed in the 
manner described by Mr. Lyell in his memoir on the marl loch of 
Forfarshire. 
Northern Drift.—Detritus differing entirely from that which covers 
Wales and Siluria, is spread over large parts of Lancashire, Cheshire, 
and N. Shropshire, ranging up to the edges of the region above 
mentioned. The materials of this drift consist of granites, porphyries, 
and other hard rocks, which have been derived from the mountains of 
Cumberland, a few perhaps from those of Scotland. The drift further 
contains much sand and clay, with many pebbles of smaller size, 
which varies exceedingly in different districts. Thus, in N. Salop, 
near the great outlier of lias, described by the author*, fragments of 
that formation are added to the mass, and as it advances to the south 
the materials become still more varied ; the fragments, however, of 
the northern granite and porphyries always existing to identify the 
drift. Its distinguishing feature is the reoccurrence at intervals of 
large blocks or boulders, of northern origin, a large proportion of 
which lie at various heights on the slopes of the mountains skirting 
the N. Welsh coal-field, and encumbering the northern flanks of the 
Wrekin and of Haughmond Hill; while a few have been propelled to 
the edge of the Silurian rocks south of Shrewsbury. They prevail in 
vast quantities in the high inland district between Wolverhampton and 
Bridgnorth, from which latitude they begin to diminish in size ; but 
coarse gravel, composed of the same materials, is prolonged south- 
wards like the tail of a delta through Worcestershire, until it dies 
away in the fine silt and gravel of the Vale of Gloucester. Not a 
fragment of any such detritus enters into the region of Welsh and Si- 
lurian drift ; but in the environs of Shrewsbury certain mounds of the 
latter are capped by clay and boulders of the northern drift, which 
is thereby shown to be of subsequent formation. The best proof of the 
recency of the epoch during which this northern drift was accumulated 
is, that it contains sea shells of existing species. These were formerly 
noticed at Preston, in Lancashire, by Mr. Gilhertson ; and by the 
author at the height of 350 feet above the sea. In Cheshire they have 
been observed by Sir P. Egerton at heights of about 70 feet+. Mr. 
Trimmer has cited similar shells on Moel Tryfanet, now ascertained to 
be 1392 feet above the sea, and has recently detected them near 
* Proceedings of the Geological Society, Vol. IT. p. 114. 
+ Ibid., Vol, Il. p- 189. t Ibid., Vol. I. p. 331. 
